My answer is: Maybe, but we’re not sure why. This was the question that motivated me to study the effectiveness of under-vine perennial cover crops for my Master’s degree thesis. Tremain Hatch, now a viticulture extension associate at Virginia Tech, found that in three consecutive years, creeping red fescue grass planted directly under the vines reduced pruning weight by 15-25% in mature Cabernet Sauvignon/101-14 vines (Hatch et al. 2011). This came with small reductions in yield while maintaining healthy vines.

The potential economic cost of reduced yields might be made up for in the money saved on summer pruning and herbicide application. Reduced vegetative growth may translate to less excess summer vine vigor, and thick turf under the vines could completely eliminate herbicide needs. This method seems to have a lot of potential for promoting healthy vine balance, but before the idea gets too far out there, it is important to know what type of stress the grass is inflicting on the vines. We need to know why the vines exhibit reduced vegetative growth, in order to know when and where this practice will work.

Getting to the root of it (literally):

In order to pinpoint how the grass impacts the vines, we had to turn to the roots, where the real competition takes place. The study took place at Virginia Tech’s Alson H. Smith Agriculture Research and Extension Center, in a mature Cabernet Sauvignon 101-14 vineyard where some vines grow with an under-vine cover crop of creeping red fescue grass, some treated with a conventional herbicide strip. We dug soil cores to one meter deep, directly under the vine trellis. We removed all the roots from this soil, and measured the total length and biomass of the vine roots at every depth. We also analyzed the nutrient availability in the soil and vines, measured soil moisture throughout the summer, and compared yield and pruning weights of the vines in both ground-cover techniques.

This is what I found:

1. Grass roots limit vine root access to essential nutrients

Several years of data in the vineyard revealed lower petiole phosphorus levels in vines grown with the cover crop, compared to vines grown with conventional herbicide application. I also found that the cover crop reduced nitrogen availability in the soil. While these levels were still in a healthy range, I hypothesized that phosphorus and nitrogen limitation might be main factors limiting vine growth in cover-cropped plots.

My results support this hypothesis. The grass cover crop roots dominated the top 6 inches of the topsoil, where the highest concentration of soil nutrients lie. As a result, the vast majority of the vine roots were pushed down into deeper soil, where nutrient concentrations (phosphorus and nitrogen) are typically much lower. The cover-cropped vines also possessed smaller overall roots systems. Root length and biomass both decreased by about 20% compared to the conventionally-grown vines. The smaller grapevine root systems may be related to smaller vine biomass above ground.

In all, it appears that the cover crop limited vine growth by restricting the vine’s access to essential nutrients that are necessary for growth. The grass roots achieved this through a combination of pushing the vine roots away from nutrient-rich soil, and reducing the overall amount of roots that can forage for available nutrients.

To our surprise, the perennial cover crop did not cause soil moisture shortage. On the contrary, the cover cropped soil retained higher moisture. This is likely because the grass created a thick mulch layer on the ground, showing evaporation. We also performed an experiment to test whether the cover crop forced the vine roots to forage deeper for water, into the deep clay soil where water extraction takes more energy. This was not the case, however – even though the cover crop pushed the vine roots toward deeper soil, the vines in both treatments still took up their water from around 1.5 feet deep.

Will the results hold up in Pennsylvania vineyards?

Our study took place over one growing season, in one vineyard in northern Virginia. While our northern Virginia vineyard has a fairly similar climate to southern PA, every vineyard is different. And, as we all know well, every growing season is different in the mid-Atlantic. But, the results of this study do suggest an important, central trend that I believe can help determine whether perennial cover crops are appropriate in a given location. In my study, the grapevine roots responded to competition with other plants by avoiding them, and moving to deeper soil. Outside of my research, I have also observed this in vineyards in which weeds or inter-row cover crops kept the grape roots out of shallow soil. Based on these observations, I can make an informed hypothesis: Across soil environments and climates, the grapevine roots are still likely to move into deeper soil in response to cover crop roots.

This is where the micro-environments of individual vineyards come into play: if grape roots are forced into deep soil in vineyards where the soil is unfavorable, the results may be an exaggerated from what we found this year. For example, if the soil already nutrient-deprived, the phosphorus decreases we observed may be exaggerated to levels that are unhealthy for the vine. On a different note, although we did not observe cover crop-induced water stress during this humid growing season, a particularly dry season may lead to stiff competition for water between the vines and grass roots. In this case, the grass may deplete limited water instead of helping the soil retain it.

That being said, I do think that perennial cover crops have strong potential to positively impact the efficiency, vine balance, and soil health of Pennsylvania vineyards. I hope to see them become a larger part of the mid-Atlantic industry in coming years.

Annie is a M.S. graduate student in Plant Biology under the direction of Dr. David Eissenstat and Dr. Michela Centinari. A special thank you goes to Dr. Tony Wolf and his team at Virginia Tech University for us of the Virginia Tech vineyard, his time, and his assistance with field measurements.